This whole story is about holes

ByPamela D. JacobsenApril 27, 2004

Do you know what makes coral, a bird's bone, and wood so strong? Hint: You can't see it, but you really need it. The answer is holes!

You may think that a completely solid object is always the strongest. Think again. When nature needs something to be strong and lightweight, holes are just the thing. Scientists call these hole-filled materials "cellular solids." Wood, bird (and some human) bones, coral, porcupine quills, and plant stalks are just a few examples of cellular solids.

Humans have learned nature's lesson and turned it into ways to make things like cars and packaging more lightweight.

Lorna J. Gibson is a professor of materials science and engineering at the Massachusetts Institute of Technology in Cambridge, Mass. She describes cellular solids as "a combination of solid rods, or plates and holes." Each rod or plate is connected to adjoining rods or plates.

Cellular solids are classified as "honeycomb" or "foamlike." In honeycomb cellular solids, the plates all go in the same direction. Plates are randomly scattered in foamlike materials.

Coral as strong as concrete

Coral is "just dead skeleton," says University of Buffalo (SUNY) biology professor Howard R. Lasker. In the ocean, coral is made up of many thousands of tiny animals called polyps. The cylinderlike polyps grow upward from their coral base. Think of a polyp as "a straw with long curtains inside," Professor Lasker says. The "curtains," called septa, are made of calcium carbonate. That's the same material seashells are made of. Some corals are about as strong as concrete, he says.

And because of the holes in between the calcium carbonate "curtains," a coral is able to grow larger than it would if it was completely solid, and do so without sacrificing strength, Lasker says.

Now imagine a structure hundreds of feet tall, thousands of yards wide, and as long as the distance from Florida to New York. That's about the size of the Great Barrier Reef off Australia's east coast. Coral reefs are found throughout the Pacific Ocean and the Caribbean.

To be strong, you have to bend a little

Wood's cells "are like a honeycomb," Professor Gibson says. A tree needs to be strong to support itself. When you look at wood under a microscope, you can see that its structure is like a honeycomb. The plates and hollows let the cells stretch or compress slightly under pressure. This flexibility helps a tree to be able to withstand strong winds.

Helping birds to stay up

Many (but not all) bird species have hollow bones. Bones that are strong and lightweight help birds fly, especially over long distances. According to Gary Ritchison, an ornithologist at Eastern Kentucky University in Richmond, "size determines whether birds have 'pneumatic' bones." A pneumatic bone is one that is largely hollow inside. The interior is "a series of little bone sections - like struts," he says. Struts are supports that connect opposite sides of bone to strengthen them.

Small songbirds such as bluejays and robins don't need hollow bones because they only weigh an ounce or two. They don't need extra weight-saving tactics. Larger, heavier birds - crows, for example - do need pneumatic bones.

Since the primary reason for hollow bones is flight, water birds like loons, grebes, or cormorants have more solid bone structures. Being heavier makes it easier for them to dive. Having hollow bones would be like trying to dive underwater while wearing a life preserver.

Have a slice of cellular solid with jam

It's easy to recognize the strength of coral, wood, or bird bone. But what about meringue, bread, or a Nestlé Aero candy bar? They're full of air spaces. They are cellular solids, too. But are they strong?

Get a piece of wood and press on it. Now take a piece of bread and do the same thing. How would you describe the strength of each? Gibson says that "strength depends upon a structure and how it is loaded."

"Loading" is an engineering concept. Think of someone sitting in a chair: That person is "loading" his weight onto the chair. How well the chair holds up tells us how strong the structure is for its weight. This idea also applies to cellular solids. But even though each object has its own strength, it isn't really possible to compare coral to bread. Or meringue to wood. Each cellular solid has its own unique properties that give it strength.

Okkyung Kim Chung is a cereal chemist and research leader who directs the United States Department of Agriculture's Agricultural Research Service. She says that breads, rolls, and cakes are "foamlike" cellular solids. Their plates are scattered randomly.

When bread dough is first mixed there isn't much air in it. Air is introduced by kneading. Kneading produces little pockets of air - let's call them cells of air - in the dough.

Yeast in the dough produces carbon dioxide. The carbon dioxide expands the existing air cells (it doesn't create new cells on its own) as the dough rises and becomes lighter or "leavened." As the dough is baked, the heat expands the air in the cells still more. Baked bread is 80 to 85 percent air, by volume, says Dr. Chung.

The baking industry today has complicated machines to determine how strong bread is (you can see a picture of such a machine at: www.gardco.com/lfra.html). But American consumers have figured out their own test for bread strength. It's called "making a peanut butter and jelly sandwich." "If the bread can hold up under the peanut butter and jelly without dropping through and onto the floor," Chung says, then the bread is strong enough.

Or take a cellular solid for a spin

Drop a metal pipe in the water, and it sinks. But what happens when you put "metal foam" into water? It floats!

Metal foam is a lightweight cellular solid that can be made from aluminum, copper, graphite, tin, carbon, zinc, lead, or titanium. Just about any metal that can be made into powdered form can become a "metal foam." It is used for objects that need to be strong and lightweight. For example, in cars, aluminum foam is placed between two solid pieces of metal to create a "sandwich panel," helping to reduce a car's weight, Gibson says.

Here's how aluminum foam is made: First, powdered aluminum and titanium hydride (titanium that's been compounded with hydrogen) are mixed together and compressed. Then the mixture is heated until the aluminum melts. As it melts, the titanium hydride releases hydrogen gas. The gas creates bubbles in the metal. The resulting metal foam is cooled. A hard foam that looks like plastic is the result. The metal foam is amazingly lightweight because 70 to 90 percent of it is holes. (To see metal undergoing a foaming process, go to: www.metalfoam.net/xraymovies.html.)

According to Lasker, "Nature always uses materials that are as strong as possible and that use as little material as possible." By copying nature, we have learned how to make our world safer and better. (See the accompanying story above for another example of a cellular solid made by humans.)

So look closely at the objects all around you. There is still a whole lot - or should I say "hole" lot? - to be learned every day!

Test the strength of a cardboard 'sandwich'

The common corrugated cardboard box is strong because it is constructed like a sandwich. There is a thin solid piece (a face) on the top and another on the bottom. In the middle is a wavy, corrugated (from the Latin word for 'wrinkled') core - a cellular solid. This core keeps the faces apart. Prof. Lorna Gibson, a materials scientist at MIT, says 'that distance between the faces gives the cardboard box its stiffness.' In fact, a solid piece of cardboard that is the same length, width, and weight will not be as stiff as the corrugated 'sandwich.'

You can prove this for yourself. You will need a corrugated cardboard box (and a grownup's permission to cut it up), scissors, a ruler, and a piece of solid cardboard that's about the same weight as the corrugated cardboard of the box.

Carefully cut two strips of corrugated cardboard out of the box. Make them about 18 inches long and as skinny as you can - an inch or so wide.

Cut one of the cardboard strips 'with the grain.' That is, cut one length of cardboard in the same direction as the wrinkles in the corrugation. When you look at the long side of this strip edge-on, you won't be able to see through it. Cut another strip 'against the grain.' When you look at this strip edge-on, you will be able to look through the little tunnels created by the wrinkled paper. Cut the solid piece of cardboard into a similar-size strip.

Which one of the three strips do you think will be strongest? Let's test them.

First, hold each one in turn at the very end and wave it. Which one seems to wiggle the most?

Try a different test. You'll need a supply of pennies for this. Make sure your strips are all the same width and length. Put one of the strips between two objects of the same height (books, perhaps). Notice how far the strips overlap on the books, as you'll want to duplicate this spacing for the other strips. Now start piling pennies, one by one, in the middle of the strip. How far does each strip bend under five pennies? Ten pennies? Does any strip collapse? So which one is the stiffest? In other words, which one bends the least under a load?

Cardboard's strength comes from the sandwich structure - but only when the height of the box goes 'with the grain.'